Description of the Related Art
The remarkable discovery by Bednorz and Muller of a new class of ceramics which exhibit superconductivity to unprecedented high temperatures (Bednorz and Muller, Zeitschrift fur Physik B 64, 189-193 (1986); Bednorz, Takashige and Muller, Europhysics Letters, 3, 379-385 (1987); Bednorz, Muller and Tagashige, Materials Research Bulletin) has triggered an unprecedented race in research laboratories around the world to investigate the properties of these materials, and to develop new materials of this type. Since then, it has been established that the polyphase material is a mixture of a green phase and a black phase, and that the superconducting phase is the black phase, an yttrium barium cuprate having the composition Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.9-x. Additional superconducting materials that have been identified include a new K.sub.2 NiF.sub.4 type perovskite, with a composition Ba.sub.2 CuO.sub.3, a ABO.sub.3 perovskite of composition YBa.sub.3 Cu.sub.2 O.sub.9, and an additional phase of composition Ba.sub.3 CuO.sub.4. The perovskite of the potassium nickel fluorite type has a tetragonal symmetry.
Following Bednorz and Muller's suggestion that a perovskite of the potassium nickel fluorite (K.sub.2 NiF.sub.4) type was responsible for the observed high Tc, workers at the University of Tokyo confirmed that the phase has indeed the tetragonal symmetry of K.sub.2 NiF.sub.4 with the stoichiometry La.sub.2-x Ba.sub.x CuO.sub.4. Such oxygen-defect perovskites can be thought of as consisting of alternating intergrowths of perovskite- and sodium chloride type layers. The structure is made up of planes of CuO.sub.6 octahedra sharing corners, separated by (La, Sr)O layers within which the La and Sr are ninefold coordinated to oxygen. As a result, the copper-oxygen bonding is distorted with the copper assuming a planar fourfold coordination with oxygen. In the La.sub.2 CuO.sub.4, all the copper is Cu.sup.2+ and the structure has a slight orthorhombic distortion. Substitution of Ba or Sr for La oxidizes some of the copper to Cu.sup.3+ by ordering of oxygen vacancies to give a mixed valence compound and decreases the orthorhombicity of the K.sub.2 NiF.sub.4 structure. A number of groups have explored the superconductivity as a function of the degree of substitution x and have found that the superconducting transition is a maximum at a value of x=0.15. This corresponds to being near to what is believed to be a metal-insulator transition.
Clarke, Advanced Ceramic Materials, Volume II, No. 3B, Special Issue (1987), page 278, has pointed out that the processing atmosphere and annealing treatments are very important in attaining good superconductivity.
The sharpest transition occurs after slow cooling in oxygen and is degraded by either annealing in too high an oxygen partial pressure or in too low an oxygen partial pressure. Quenching from too high a temperature, even in oxygen, adversely affects the transition.
The intercalation of oxygen and the effect of annealing treatments as well as quenching can be interpreted in structural terms primarily as affecting the oxygen occupancy on the copper plane lying between the barium ions. This is based on the results of neutron diffraction and supports the idea that there is an optimum oxygen stoichiometry for high temperature superconductivity. The prevailing view at the present time is that it is the perfection of the CuO.sub.2 "ribbons" or "chains" created by oxygen vacancy ordering in the basal plane that is critical for high values of the superconducting transition temperature Tc. In this interpretation, deviations from the ideal structure caused by the addition (high oxygen pressure) or removal (low oxygen pressure) of oxygen from the copper-oxygen plane between the barium ions will result in lowering of the transition temperature.
One of the consequences of the processing atmosphere and temperature is that the orthorhombic distrotion of the unit cell alters. Above a certain temperature the unit cell becomes tetragonal and samples quenched from this tetragonal state do not exhibit 90.degree. K. superconductivity. Using neutron diffraction, it has been established that the orthorhombic-to-tetragonal transition occurs when the oxygen stoichiometry falls to a value of 6.5, and that the temperature at which this transition occurs depends on the oxygen partial pressure of the experiment. In terms of the unit cell description, as oxygen leaves the Cu--O--Cu chains in the orthorhombic cell the stoichiometry falls until at a value of 6.5 the oxygen ions disorder between sites in the basal plane in the a and b axes making them equivalent and thereby causing the cell to become tetragonal. Thus, if a sample is quenched from above the phase transition temperature, the oxygen ions will not form the Cu--O--Cu chains and superconductivity will be lost.
A rather dramatic illustration of the importance of processing is that the single phase La.sub.2 CuO.sub.4 material has been known for many years to be insulating, yet recently groups at both IBM Almaden and Grenoble have shown that it can be converted to exhibit superconductivity with an onset as high as 40.degree. K. by annealing in air at about 950.degree. C. Furthermore, the superconductivity can be reversibly created and destroyed by alternating heat treatments in an oxidizing and reducing atmosphere.
Engler et al Chemistry of High Temperature Superconductors, (a publication of the American Chemical Society (1987)) Chapter 25, Processing, Structure, and High-Temperature Superconductivity, has commented that "Quite early it was recognized that the superconducting properties of Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.9-y depended on the processing conditions (Grant et al Phys. Rev. B1 1987, 35, 7242; Engler et al J. Amer. Chem. Soc, 1987, 109, 2848). This can be understood by considering the idealized structure of Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.9-y which involves an ordering of Ba--Y--Ba in triplets (5) and in principle, can accommodate nine oxygens. Experimentally, however, this value has been determined to be approximately seven. How the oxygens distribute themselves in the structure turns out to be very dependent on processing conditions and is the key to achieving the highest and sharpest superconducting transitions."
An example of the preparatory procedure that has been utilized is given by Poeppel et al, Chapter 24, of the same text. Poeppel et al tried to develop a reliable procedure to fabricate YBa.sub.2 Cu.sub.3 O.sub.7. The solid state reaction method was used. Barium carbonate BaCO.sub.3, Y.sub.2 O.sub.3 and CuO were wet-milled, and the excess fluid then evaporated off. Other precursor materials such as BaO and Cu.sub.2 O have also been used. Next the raw powder is subjected to calcination, a step which Poeppel et al state "has proved to be surprisingly critical in the production of good superconductive material."
Apparently, the BaCo.sub.3 --CuO system can preferentially form a eutectic at about 875.degree. C., causing a phase separation that inhibits the formation of the desired 1-2-3 compound. Also, there seems to be a difference in relative reaction rates. In binary tests, the reaction of BaCO.sub.3 with CuO to form BaCuO.sub.2 is much faster than the reaction of Y.sub.2 O.sub.3 with either CuO (forming Y.sub.2 Cu.sub.2 O.sub.5) or BaCO.sub.3 (forming Y.sub.2 Ba.sub.4 O.sub.7). The difference in reaction rates again leads to phase separation. Improperly calcined material is very hard and shows excessive grain growth. To overcome the problems observed, two empirical procedures have been developed.
The first procedure uses a long-term precalcine (.gtoreq.24 h) at 850.degree. C. to decompose the carbonates below the eutectic and form the 1-2-3 compound as the major phase. This material is then lightly ground and final-calcined at 950.degree. C. for about 2 h in a laboratory kiln which reaches a temperature in about 15 min.
A second method is a slight variation of the first. The raw powders are taken to a temperature of 950.degree. C. for 2 to 6 h in the same quick-firing kiln mentioned above, cooled and reground, and the procedure repeated for a total of three calcinations. The powders are then checked for phase compositions by x-ray diffraction (XRD). Chemical analysis was performed using inductively coupled plasma-atomic absorption analysis and has confirmed that the composition of these powders is correct to within the accuracy of the device.
An interesting aspect of both of these procedures is that a fast heat-up rate on the final 950.degree. C. calcine appears to be crucial to success. A second puzzling aspect of this system is that if one tries to sinter powders at temperatures of 950.degree. to 975.degree. C. which have only been calcined to 850.degree. C., poor superconducting materials will result, even though XRD shows the powder to be pure 1-2-3. It appears that calcination temperatures must be greater than 900.degree. C. to ensure good properties.
Following formation of the YBa.sub.2 Cu.sub.3 O.sub.7 adding calcination, the particle size is reduced, and the resulting powder is then sintered to form the final dense ceramic. The powder is formed into the desired shape or configuration, and then sintered under a flowing oxygen atmosphere to encourage equilibrium oxygen stoichiometry.
Calcination of a stationary bed of material is not a satisfactory method. It does not provide precise temperature control throughout the bed, nor is it possible to obtain a uniform application and distribution of heat, or good contact between the material and the clacination atmosphere. Poor temperature control can cause melting, and the formation of undesirable crystal phases. (Cima and Rhine, "Powder Processing for Microstructural Control in Ceramic Superconductors", Advanced Ceramic Materials, Vol. 2, No. 3B, Special Issue, 1987, p. 329).
Nonhomogeneous heating does not allow uniform decomposition and reaction of the material (Ibid. p. 330).
Poor air or oxygen contact restricts the oxygen content of the material, which reduces downstream processing flexibility (Clarke, Ibid., p. 281).
Producing large quantities of uniform material is particularly difficult, because of these problems.